Furnace cold air return serving up crawlspace air

Question

My house has a partial basement and a crawlspace. The forced air furnace is in the basement, and ducts run through the crawlspace to distribute air to the house. In the summer, when we don't use the furnace, our living room, where the cold air return is, starts to smell like dirt. Like the crawlspace. I think the issue is that certain atmospheric conditions will cause a pressure or temperature differential that makes air flow up and out of the crawlspace via the cold air return.

This must be an issue for other houses with crawlspace duct work. Is there a standard way that HVAC pros mitigate this? Would it help to add more ventilation to my crawlspace (it has very poor ventilation currently)?

The easiest solution would be to block off the cold air return when it's not in use, but I'm the kind of moron who would probably forget about it and turn on the furnace while it's still blocked.

Answer 1

Fact: ducts leak

Just about all ductwork leaks to some extent; in fact, without specialized installation techniques, leakages of around 20% can be expected. Combine that with the fact that the furnace is sucking on the return ducts, and you get a recipe for air infiltration from the crawlspace.

Venting the crawlspace might help, but sealing the ducts is better all-around, and rerouting them is even better yet

As far as the smell goes, venting the crawlspace might help, but a better bet is to have the ducts in the crawlspace redone in a sealed fashion -- it's possible to get the leakage down to a few % this way. Of course, it's even better to reroute the ductwork so it's in the conditioned space instead, but not all houses lend themselves to that approach.